Discoid eczema (nummular eczema) can be differentiated from other types of dermatitis during clinical examination by its characteristic presentation of well-demarcated, round or oval plaques that are often coin-shaped and covered with scaling and crusting. These lesions typically appear on the limbs and trunk and may be intensely pruritic. Unlike atopic eczema, which often involves flexural areas and has a more diffuse pattern, discoid eczema presents as discrete, localized patches. The lesions may also show central clearing with peripheral scaling, which helps distinguish them from other eczematous conditions such as seborrheic dermatitis or contact dermatitis NICE CG57. In contrast to allergic or irritant contact dermatitis, discoid eczema lesions are usually symmetrical and not confined to areas of direct contact with irritants or allergens. Additionally, the chronicity and recurrent nature of discoid eczema, with lesions that can persist for weeks to months, further aid differentiation Leung et al. 2020.
On examination, the absence of typical features of other dermatitis types—such as the greasy scales and involvement of seborrheic areas in seborrheic dermatitis, or the sharply demarcated erythema corresponding to allergen exposure in contact dermatitis—supports the diagnosis of discoid eczema. The clinical history of recurrent coin-shaped plaques, often exacerbated by dry skin or environmental factors, also assists in differentiation NICE CG57 Leung et al. 2020.